1. Field of the Invention
This invention relates to spectrometers and in particular to atomic absorption spectrometers which use the Zeeman effect for background correction.
2. Description of the Related Art
Atomic absorption spectrometers are used to determine the amount or concentration of an element in a sample material. The sample which is to be analysed is atomized to produce a cloud of atoms for analysis. A beam of light from a light source is directed through the cloud of atoms and is detected. Light is absorbed by the atoms in the cloud at wavelengths which are characteristic of the element being determined. However, absorption of the measuring light beam is not only caused by the atoms which are to be analysed but also by other sources which produce background absorption which has to be corrected in order to provide accurate measurement of the atomic absorption.
The Zeeman effect is used for background correction. This technique uses a magnetic field which is applied to the atoms in the cloud or in the light source to split and shift the absorption lines of the atoms or emission lines of the source. The shifting of the absorption lines of the atoms ensures that they no longer coincide with the spectral lines of the measuring light beam and this permits discrimination between atomic absorption caused by the atoms and background absorption.
Spectrometers which employ the Zeeman effect utilise an electromagnet which is typically powered by sinusoidal current from power mains, as shown in FIG. 1A. More recently a square wave is driven onto coils of the electromagnet and background absorption measurements are made when the electromagnet is turned on and measurements of total absorption (atomic plus background) are made when the electromagnet is turned off. FIG. 1B.
We have discovered that conventional atomic absorption spectrometers utilising the Zeeman effect produce a magnetic field which changes slowly between switching on and off of the electromagnetic so that there is a significant time delay between the period in which atomic absorption is measured and the period in which total absorption is measured thereby resulting in a significantly large dead time between measurements.
The slow change in magnetic field is illustrated with reference to FIG. 1A and 1B. Background absorption is measured in time period A. Total absorption is measured in time period B. The time period C between periods A and B is the dead time which is not normally used for measurement.
We have identified a number of significant problems which result from the dead time referred to above.
The disadvantages may be summarised as follows:
1. Poor background correction due to the significant time difference between making background absorption measurement and a total absorption measurement in situations where the background absorption changes with time. Since the background absorption changes with time the background absorption taking place at the times of the background absorption measurement and the total absorption measurement can be significantly different thereby giving incorrect background correction. PA1 2. Measurement time efficiency is poor due to the delay caused by the changing magnetic field in which no atomic absorption measurement or background measurement can be made. PA1 3. Since the time during which measurements can be made is small, high peak lamp currents must be used in order to achieve acceptable noise levels. The high peak lamp currents result in poorer sensitivity of the spectrometer and reduced 1 amp life. PA1 a) To obtain the necessary magnetic flux of approximately 1 tesla, a very high number of ampere turns (typically 15,000 to 20,000) is required. PA1 b) High ampere turns requires a large number of turns on the electromagnet and a large current. PA1 c) A large number of turns produces a large L (Inductance) value which reduces (limits) the rate of change of current for a given voltage.